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The Java Language Specification [JLS 2011] §8.9, "Enums" does not specify the use of ordinal() in programs. However, using attaching external significance to the ordinal() method to derive the value associated with an  value of an enum constant is error prone and should be avoided.

Noncompliant Code Example

This noncompliant code example declares enum Hydrocarbon and uses its ordinal() method to provide the result of the getNumberOfCarbons() method.

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Although this noncompliant code example currently works, its maintenance is likely to be problematic. If the enum constants were reordered, the getNumberOfCarbon() method would return incorrect values. Furthermore an additional BENZENE constant could not be added to the model because it has 6 carbons, but the ordinal value 6 is already taken.

Compliant Solution

In this compliant solution, enum constants are explicitly associated with the corresponding integer values for the number of carbon atoms they contain. Thus, the ordinal() method is no longer involved required in knowing the number of carbon atoms for each value.

enum Hydrocarbon {
  METHANE(1), ETHANE(2), PROPANE(3), BUTANE(4), PENTANE(5),
  HEXANE(6), HEPTANE(7), OCTANE(8), NONANE(9), DECANE(10);

  private final int numberOfCarbons;

  Hydrocarbon(int carbons) { this.numberOfCarbons = carbons; }

  public int getNumberOfCarbons() {
    return numberOfCarbons;
  }
}

Applicability

Use of ordinals to derive integer values reduces the program's maintainability and can lead to errors in the program.

Related Guidelines

ISO/IEC TR 24772:2010: "Enumerator Issues [CCB]"

Bibliography

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